High-voltages such as up to 1.5 kV occur in photovoltaic installations through the series connection of photo-elements to module strings, from module strings to photovoltaic modules, and from photovoltaic modules to module strings. The totality of these series-connected components is designated as a photovoltaic generator of the photovoltaic installation. The DC voltage produced by the photovoltaic generator is commonly routed through connector lines to a voltage converter such as an inverter. The inverter typically generates an output voltage having the voltage and frequency of a conventional AC network. This output voltage can be routed to electric loads or stored in a voltage supply system.
Faults can arise due to the high voltages on the components of the photovoltaic generator and particularly on the connector lines. The faults include insulation faults and arc discharges which can give rise to additional hazards that should be rapidly detected and quenched.
In principle, series arc discharges and parallel arc discharges can be distinguished from one another depending on whether the arc discharge occurs along a connector line or between two connector lines. This distinction will be discussed in greater detail with the aid of FIGS. 2 and 3.
FIGS. 2 and 3 each illustrate a conventional photovoltaic installation. In particular, as explained below, FIG. 2 illustrates the conventional photovoltaic installation in which a series arc discharge LS is present and FIG. 3 illustrates the conventional photovoltaic installation in which a parallel arc discharge LP is present. The conventional photovoltaic installation includes a photovoltaic generator PG. As shown schematically in FIGS. 2 and 3, photovoltaic generator PG is formed from a simple series connection of four photovoltaic modules M1, M2, M3, M4. Actual embodiments of the photovoltaic generator can be significantly more complex. Photovoltaic generator PG is connected to the input of an inverter WR through two electrical connector lines V1, V2.
As the output voltage of photovoltaic generator PG typically has a high value, faults in connector lines V1, V2 can lead to arc discharges. DC voltage arc discharges are not self-quenching and can thus persist for a relatively long time unless suitable countermeasures are taken. This presents the danger of fire and personal injury.
The conventional photovoltaic installation further includes a circuit breaker TS and a short-circuiting switch KS. Circuit breaker TS and short-circuiting switch KS are for extinguishing series and parallel arc discharges LS, LP. Circuit breaker TS runs in series with one of connector lines V1, V2 as shown in FIGS. 2 and 3. Short-circuiting switch KS is connected across connector lines V1, V2 as shown in FIGS. 2 and 3. Circuit breaker TS and short-circuiting switch KS are activated or controlled by a suitable device (not shown) upon detection of an arc discharge LS, LP. Circuit breakers such as circuit breaker TS and short-circuiting switches such as short-circuiting switch KS are common means used for extinguishing series and parallel arc discharges.
As indicated above, FIG. 2 illustrates the conventional photovoltaic installation in which a series arc discharge LS is present. Series arc discharge LS generally arises from a poorly conducting section of either of connector lines V1, V2. In particular, series arc discharge LS may be ignited when high-voltage electricity is applied over the poorly conducting section of connector line V1, V2. Series arc discharge LS is maintained by the arc discharge current. If the current flowing to the poorly conducting section of connector line V1, V2 is interrupted, then series arc discharge LS extinguishes. Circuit breaker TS can enable such an interruption assuming that the current of series arc discharge LS flows through circuit breaker TS.
However, if a conducting connection is present between connector lines V1, V2 in front of circuit breaker TS such as in the region of short-circuiting switch KS as shown in FIGS. 2 and 3, then opening circuit breaker TS is ineffective in extinguishing series arc discharge LS. For this reason, short-circuiting switch KS is also not suitable for extinguishing series arc discharge LS. This is particularly the case when the closed short-circuiting switch KS does not reduce the arc current, but amplifies the arc current, which happens in most cases.
As indicated above, FIG. 3 illustrates the conventional photovoltaic installation in which a parallel arc discharge LP is present. Parallel arc discharge LP arises between connector lines V1, V2. Short-circuiting switch KS is usually effective in controlling parallel arc discharge LP. Short-circuiting switch KS causes low resistance bridging of the arc gap through which the voltage across parallel arc discharge LP drops to such a low value that the electric arc is extinguished. For this to occur, the sections of connector lines V1, V2 between the position of parallel arc discharge LP and short-circuiting switch KS have to remain intact. If these sections are damaged by closing short-circuiting switch KS, in particular by parallel arc discharge LP, then this function can fail. Circuit breaker TS is not effective here as circuit breaker TS is not connected in a section of connector lines V1, V2 between parallel arc discharge LP and photovoltaic generator PG.
In summary, in the conventional photovoltaic installation shown in FIGS. 2 and 3, the effectiveness of circuit breaker TS and short-circuiting switch KS for extinguishing arc discharges LS, LP depends on: (i) the type (serial or parallel) of arc discharge LS, LP with respect to the components of the photovoltaic installation; and (ii) any damage that arc discharge LS, LP has already caused by deploying the extinguishing measures on connector lines V1, V2. If these factors are unknown and are not taken into consideration, then a non-negligible probability exists that the measures described for extinguishing electric arcs will not be effective.
Optical sensors are used for detecting electric arcs. The optical sensors are configured to detect optical radiation that is typical of an electric arc. The position where an arc discharge occurs cannot normally be predicted. Thus, a relatively large amount of sensors are used for monitoring a spatially extended photovoltaic installation and this involves expense.
FIG. 4 illustrates a photovoltaic installation according to the operating principle suggested in DE 20 2010 008 494 U1. The photovoltaic installation includes a photovoltaic generator PG formed from a plurality of photovoltaic modules M1, M2, M3, M4 connected in series. Photovoltaic generator PG generates a DC voltage which is routed through connector lines V1, V2 to an inverter WR. The photovoltaic installation further includes an enabling signal generator FSG. Enabling signal generator FSG produces an enable signal FS. Enable signal FS is modulated on either of connector lines V1, V2. Enable signal FS controls switching elements S1, S2, S3, S4 of respective photovoltaic modules M1, M2, M3, M4. Switching elements S1, S2, S3, S4, in their opened state, are arranged in parallel to respective photovoltaic modules M1, M2, M3, M4.
The photovoltaic installation shown in FIG. 4 further includes a failure sensor S. DE 20 2010 008 494 U1 describes failure sensor S as being a smoke detector or a heat sensor. Upon detecting a failure, failure sensor S controls enabling signal generator FSG to stop emitting enable signal FS. Consequently, switching elements S1, S2, S3, S4 transition from their opened state into their closed state as enable signal FS has been removed. In their closed state, switching elements S1, S2, S3, S4 short circuit the subordinate photovoltaic modules M1, M2, M3, M4 and thus convert to a secure state.
DE 20 2010 008 494 U1 does not describe a failure or breakdown that can lead to arc discharges. Accordingly, fault sensor S is not described as being configured to detect arc discharges. In addition, information is lacking on how such a sensor could be made. Likewise, the problem of detecting and extinguishing different types of arc discharges (series and parallel) is not addressed.